in peripheral resistance shifts the vascular function curve more than it does the cardiac function curve, equilibrium point B is below and to the left of point A; that is, both cardiac output and Pv diminish. Conversely, if the cardiac function curve is displaced more than the vascular function •Fig. 19.11 Anincreaseinperipheralresistanceshiftsthecardiacandvascularfunctioncurvesdownward.Atequilibrium,cardiacoutputisless(pointB)whenperipheralresistanceishighthanwhenperipheralresistanceisnormal(pointA).
•Fig. 19.12 Simplifiedmodelofthecardiovascularsystemthatconsistsoftheleftventricle(LV)andrightventricle(RV),systemicvascularresistance(Rs)andpulmonaryvascularresistance(Rp),systemicarterialandvenouscompliance,andpulmonaryarterialandvenouscompliance.PsaandPsvarethepressuresinthesystemicarteriesandveins,respectively;PpaandPpvarethepressuresinthepulmonaryarteriesandveins,respectively.
curve, point B falls below and to the right of point A; that is, cardiac output decreases, but Pv rises.
A More Com﻿plete Theoretical Model: The Two-Pum﻿p System﻿
The preceding discussion shows that the interrelationships between cardiac output and Pv are complex, even in an oversimplified circulation model that includes only one pump and just the systemic circulation. In reality, the cardiovascular system includes the systemic and pulmonary circulations and two pumps: the left and right ventricles. Thus the interrelationships among ventricular output, arterial pressure, and atrial pressure are much more complex.
Fig. 19.12 depicts a more complete (but still oversimplified) cardiovascular system model that has two pumps in series (the left and right ventricles) and two vascular beds in series (the systemic and pulmonary vasculature). The series arrangement requires that the flow pumped by the two ventricles be virtually equal to each other over any substantial period; otherwise, all the blood would ultimately accumulate in one or the other of the vascular systems. Because the cardiac function curves for the two ventricles differ substantially, the filling (atrial) pressures for the two ventricles must differ appropriately to ensure equal stroke volumes (see
Fig. 18.13